Terror of the rock pools
Compatibility is a major concern with rock pool aquariums. Basically it is the nature of rock pool inhabitants to eat other rock pool inhabitants, if they can't eat each other for what ever reason then they will fight over territory or food. Because of this careful thought should be given to any proposed stocking before hand.
The above video clip shows just how quickly things can go wrong. We had set up this tank purely for taking photos, the shanny had been in the tank for a matter of hours and the porcelain crab for a couple of days.
The shanny obviously settled down much quicker than we anticipated and tried to make a meal out of the crab, Fortunately it failed and tired while trying and the crab although probably frightened was physically unscathed.
There are steps that you can take in order to minimise the risks.
1, Keep animals of around a similar size, this makes it difficult (in most cases) for one to eat another.
2, Don't keep predator and prey in the same tank, sooner or later the predator will win!!!
3, Make the tank look natural and provide lots of cover so that the fish and inverts aren't always within sight of each other.
4, Don't over stock, this is important for many reasons not just for compatibility.
5, Don't keep shy timid fish with really bold feeders because they won't get their share of the food and will slowly lose condition and their health will suffer.
Rockpool tanks are quite dynamic and just because it works in the beginning doesn't guarantee that it will work in six months time. Fish grow, sometimes quite quickly and as they do they may become more dominant and try to control all the food. Always be ready to intervene before it becomes a real problem.
Of all the things to consider when stocking a temperate marine aquarium the most important is biological filtration and is the main limiting factor. Filter bacteria work more slowly in cooler conditions so as a rough guide try to allow 2 imp gall of water for every inch of fish (9 litres/2.5cms) once the tank is fully mature and at least 6 months old. The stocking rate should be halved if the tank is new (even if it has completed a cycle).
it is no use putting a filter feeder or fish which eat food of a tiny size in a very well filtered tank as there will be little or no food available to them, as already mentioned, bold feeders like mullet shouldn't be kept with fish which are quite shy like small wrasse because the timid fish can't or won't compete for food.
Don't try to keep fish which are normally found in the shallow sea as opposed to those which truly live in rock pools in an un chilled aquarium. They aren't a adaptable as the real rock pool species and they will become really stressed if the water heats up to quickly under the aquarium lighting. If you do have a chiller then you can keep these fish and you can still keep the real rock pool fish too.
Some rock pool fish can eventually reach a substantial size. Make sure that you do some research about the fish and inverts that you intend to keep. This is especially important if you collect specimens when on holiday because a 2 - 4 inch mullet might look good in your tank but it will soon turn into a fish of 2ft and if you aren't near the coast where the fish came from then you have a problem in the making. This applies to a good number of fish which can be found in rock pools.